1. Field of the Invention
The present invention relates to an enzymatic method for measuring creatine or creatinine in serum, as well as to a reagent for use in these measurements.
2. Description of the Prior Art
Today, the measurement of creatine or creatinine in serum and urine is an important item of clinical test for diseases of kidney and muscles. In most cases, the measurement is carried out according to the Jaffe method. This method is based on a non-specific chemical reaction and the results obtained by this method can be erroneous. A variety of enzymatic methods have recently been developed with the aim of overcoming this disadvantage of Jaffe method, of which one example is a color-forming determination by the following enzymatic reactions (Japanese Patent Publication No. 60-3480): ##STR1##
The above-mentioned method is advantageous in that it is not influenced by various substances affecting Jaffe method such as protein, ketone compound, cephalosporin type antibiotics, etc. However, this method is disadvantageous in that it makes a plus error (abnormally high result of measurement) because sarcosine oxidase acts not only upon sarcosine but also upon N-ethylglycine which is a metabolic product of therapeutically administered drug (e.g. lidocaine) owing to its substrate specificity. The present inventors conducted many studies with the aim of overcoming the disadvantage of the prior enzymatic measurement to find that creatine or creatinine in a sample can be determined with a high accuracy if N-ethylglycine present in the sample is previously decomposed with an enzyme exhibiting a high activity on N-ethylglycine and thereafter the creatine or creatinine is measured. Based on this finding, the present invention was accomplished.